1. Field of the Invention
This invention relates to an improved servo-valve, and more particularly to such a valve useful in a fuel injector equipped with a pressure booster.
2. Description of the Prior Art
Known stroke-controlled high-pressure accumulator injection systems (common rail) can be used to inject fuel in direct-injecting internal combustion engines. These injection systems are distinguished by the fact that the injection pressure can be adapted to the load and speed of the engine. A high injection pressure is required in order to reduce emissions and to achieve high specific outputs. Since the achievable pressure level in high-pressure fuel pumps is limited for strength reasons, a further pressure increase in high-pressure injection systems (common rail) can be achieved by means of pressure boosters in injectors.
DE 101 23 913 has disclosed a fuel injection apparatus for internal combustion engines, having a fuel injector that can be supplied from a high-pressure fuel source. A pressure boosting device that has a movable pressure booster piston is connected between the fuel injector and the high-pressure fuel source. The pressure booster piston divides a chamber that can be connected to the high-pressure fuel source from a high-pressure chamber connected to the fuel injector. The fuel pressure in the high-pressure chamber can be varied by filling a return chamber of the pressure boosting device with fuel or by emptying fuel from the return chamber. The fuel injector has a movable closing piston for opening and closing injection openings; the closing piston protrudes into a closing pressure chamber. Fuel pressure can be exerted on the closing piston to produce a force that acts on the closing piston in the closing direction. The closing pressure chamber and the return chamber are constituted by a combined closing pressure/return chamber; all of the partial regions of the closing pressure/return chamber are permanently connected to one another to permit the exchange of fuel. A pressure chamber is provided for supplying fuel to the injection openings and for exerting a force on the closing piston in the opening direction. The high-pressure chamber is connected to the high-pressure fuel source so that aside from pressure fluctuations, at least the fuel pressure of the high-pressure fuel source can continuously prevail in the high-pressure chamber. The pressure chamber and the high-pressure chamber are constituted by a combined injection chamber whose partial regions are permanently connected to one another to permit the exchange of fuel.
In fuel injectors, servo-valves can be used as on/off valves, which have a one-piece servo-valve piston whose control cross sections are embodied in a seat/slider design. In servo-valves of this kind, a significant amount of wear on the slider surfaces can occur since only short overlap lengths can be achieved. In addition, in servo-valves with a seat/slider design, high demands are placed on manufacturing precision, particularly with regard to the position of the control edges of the servo-valve piston in relation to each other.